Roller cone rock drill bits and fixed cutter drill bits are commonly used in the oil and gas industry for drilling wells. FIG. 1 shows one example of a conventional drilling system drilling an earth formation. The drilling system includes a drilling rig 10 used to turn a drill string 12 which extends downward into a well bore 14. Connected to the end of the drill string 12 is a bottomhole assembly, which includes at least a drill bit 20 that cuts through and breaks up earth formation as it is rotated.
One example of a roller cone-type drill bit is shown in FIG. 2. Roller cone bits 20 typically comprise a bit body 22 having an externally threaded connection at one end 24, and a plurality of roller cones 26 (usually three as shown) attached to the other end of the bit and able to rotate with respect to the bit body 22. Attached to the cones 26 of the bit 20 are a plurality of cutting elements 28 typically arranged in rows about the surface of the cones 26. The cutting elements 28 are typically tungsten carbide inserts, polycrystalline diamond compacts, or milled steel teeth.
Significant expense is involved in the design and manufacture of drill bits. Therefore, having accurate models for simulating and analyzing the drilling characteristics of bits can greatly reduce the cost associated with manufacturing drill bits for testing and analysis purposes. For this reason, several models have been developed and employed for the analysis and design of fixed cutter bits. These fixed cutter simulation models have been particularly useful in that they have provided a means for analyzing the forces acting on the individual cutting elements on the bit.
However, roller cone bits are more complex than fixed cutter bits in that each roller cone independently rotates relative to the rotation of the bit body about axes oblique to the axis of the bit body. Additionally, the cutting elements of the roller cone bit deform the earth formation by a combination of compressive fracturing and shearing, whereas fixed cutter bits typically deform the earth formation substantially entirely by shearing. Because each roller cone independently rotates about an axis oblique to the axis of the bit body, a conventional rock bit may experience unbalanced lateral forces (radial forces) that cause the rock bit to gyrate or laterally bounce about the bottom hole and impact the wall of the wellbore during drilling. This type of bit motion is generally referred to as bit gyration or “whirling.” Bit whirling is an undesirable performance characteristic, because it results in inefficient drilling of the bottomhole and can potentially damage the bit prematurely.
Accurate analysis of the drilling performance of roller cone bits requires more complex models than for fixed cutter bits. Until recently, no reliable roller cone bit models had been developed which could take into consideration the location, orientation, size, height, and shape of each cutting element on the roller cone, and the interaction of each individual cutting element on the cones with earth formations during drilling.
In recent years, some researchers have developed a method for modeling roller cone cutter interaction with earth formations. See D. Ma et al, The Computer Simulation of the Interaction Between Roller Bit and Rock, paper no. 29922, Society of Petroleum Engineers, Richardson, Tex. (1995). However, methods have not been specifically developed for optimizing the performance of drill bits, particularly, roller cone bits, in drilling earth formations to analyze bit performance with respect to the lateral (radial) force of the bits. To produce new and improved bits designed to exhibit desirable drilling characteristics, such as minimized bit whirl or a later walk tendency, such methods are desired and may be used. Bit specifically designed to exhibit reduced whirling tendencies may drill more efficiently with increased longevity maximizing the drilling performance of a given bit.